Superconducting coils for the generation of strong magnetic fields are often operated in a manner known as persistent current mode. In this mode, the magnet winding is shorted to a closed superconducting circuit, and the current flows almost indefinitely without a substantial resistance in the circuit. This approach results in a magnetic field which is highly stable over time. Stable, homogeneous and strong magnetic fields are for example needed for magnetic resonance imaging and for nuclear magnetic resonance spectroscopy, where magnetic flux densities between 0.5 T and 20 T are common. The magnetic coils for these applications are usually charged via an external circuit and then disconnected from their external power supply. Afterwards, the current flows through the coil in an almost loss-free mode, and the resulting magnetic field does not suffer from the noise contributions of an external power source. In known magnetic resonance coils, one or more superconducting wires are wound onto one or more coil carriers. Several such individual coil sections are usually connected by a superconducting joint. A joint resistance of less than 1012 Ohm per joint is usually required for persistent mode operation. For some applications, a joint resistance as low as 1015 Ohm is even necessary. Classical low-temperature superconductors such as Nb—Ti and Nb3Sn with transition temperatures below 23 K can be joined by superconducting contacts with existing technologies such as soldering with low-temperature superconducting material. The same technique can be used to provide superconducting switches. These switches are part of the closed circuit of the magnetic coil, but for charging the coil they are heated to a normal-conducting state. After charging from an external current source, the switch is again cooled to a superconducting state and the current source is disconnected. High-temperature superconductors are superconducting materials with a transition temperature above 25 K. They are attractive for use in superconducting magnetic coils mainly for two reasons: Low-temperature superconductors require cooling by liquid helium, which is expensive and of limited supply. High-temperature superconductors such as MgB2 and oxide ceramic superconductors can be cooled with cheaper, higherboiling cryogens. Alternative cooling concepts based on liquid helium are also available, where the amount of helium consumption is reduced, when the coil is operated at a temperature around or above 10K. A second advantage of materials with higher transition temperature is that they usually have a higher intrinsic critical current density and higher critical magnetic fields. Therefore, it is easier to achieve very high magnetic flux densities with high-temperature superconductors. A difficulty of using high-temperature superconductor materials for persistent-mode magnetic coils is that superconducting joints are not readily available.
Connections of MgB2 wires have previously been described by joining the wire ends by additional MgB2 material provided in the joint region.
US 2009/0105079A1 describes a superconductive connection for the stripped end pieces of two superconducting wires. MgB2 is used as a superconducting contact material, which is filled into a sheath or bushing that the two wire ends are also inserted to. The cross-section of the sheath or bushing is then reduced in order to densify the contact material. EP 2436087 also describes a superconductive connection for two wires with MgB2 filaments embedded in a normally conducting matrix material. The stripped filaments and MgB2 or its base materials are inserted into a sleeve or bushing to form a superconducting contact. The filaments are also sheathed with a barrier layer, which is at least partially stripped in the contact region. EP 2383969 also describes a method for producing a connection structure between two superconductors, in particular MgB2 conductors. Here, a substance which lowers the melting temperature of magnesium is admixed to a material mixture of magnesium and boron and the exposed ends of the core wires are brought in contact with the material mixture, which is made to react with MgB2 in situ.
WO2009127956A1 describes a superconducting joint that structurally binds a first superconducting segment to a second superconducting segment. The superconducting segments each include corresponding areas containing a granular superconducting substance formed by a first element and a second element. The superconducting joint includes a solid non-superconducting binding formed from a source of the first element and a source of the second element combined to produce the granular superconducting substance around the solid non-superconducting binding to form the superconducting joint.
However, these methods described so far have not provided a way of obtaining superconducting joints which have a joint resistance below 1012 Ohms measured with significant current of 10 A or higher, at temperatures of 10 K or higher and at background magnetic fields of IT or more.